This invention relates to the casting of steel strip. It has particular application to continuous casting of thin steel strip in a twin roll caster.
In twin roll casting, molten metal is introduced between a pair of counter-rotated horizontal casting rolls, which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting thin steel strip in a twin roll caster, the molten steel in the casting pool will generally be at a temperature of the order of 1500° C. and above, and therefore high cooling rates are needed over the casting roll surfaces. It is important to achieve a high heat flux and extensive nucleation on initial solidification of the steel on the casting surfaces to form the metal shells. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry so that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature so as to form a substantially liquid layer at the interface between the molten metal and the casting surface. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular International Application AU 99/00641 discloses that a random texture of peaks and troughs can enhance initial solidification by providing potential nucleation sites distributed throughout the casting surfaces. We have now determined that nucleation is also dependent on the presence of oxide inclusions in the steel melt and that, surprisingly, it is not advantageous in twin roll strip casting to cast with “clean” steel in which the number of inclusions formed during deoxidation has been minimized in the molten steel prior to casting.
Steel for continuous casting is subjected to deoxidation treatment in the ladle prior to pouring. In twin roll casting, the steel is generally subjected to silicon manganese ladle deoxidation. However, it is possible to use aluminum deoxidation with calcium addition to control the formation of solid Al2O3 inclusions that can clog the fine metal flow passages in the metal delivery system through which molten metal is delivered to the casting pool. It has hitherto been thought desirable to aim for optimum steel cleanliness by ladle treatment and minimize the total oxygen level in the molten steel. However we have now determined that lowering the steel oxygen level reduces the volume of inclusions, and if the total oxygen content and free oxygen content of the steel are reduced below certain levels the nature of the intimate contact between the steel and roll surfaces can be adversely effected to the extent that there is insufficient nucleation to generate rapid initial solidification and high heat flux. Molten steel is trimmed by deoxidation in the ladle such that the total oxygen and free oxygen contents fall within ranges which ensure satisfactory solidification on the casting rolls and production of a satisfactory strip product. The molten steel contains a distribution of oxide inclusions (typically MnO, CaO, SiO2 and/or Al2O3) sufficient to provide an adequate density of nucleation sites on the roll surfaces for initial and continued solidification and the resulting strip product exhibits a characteristic distribution of solidified inclusions and surface characteristics.
There is provided a method of casting steel strip comprising:
assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip;
introducing molten low carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 70 ppm, usually less than 250 ppm, and a free-oxygen content of between 20 and 60 ppm;
counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and
forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.
There is also provided a method of casting steel strip comprising:
assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip;
introducing molten low carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 100 ppm, usually less than 250 ppm, and a free-oxygen content between 30 and 50 ppm;
counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and
forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.
The total oxygen content of the molten steel in the casting pool may be about 200 ppm or about 80–150 ppm. The total oxygen content includes free oxygen content between 20 and 60 ppm or between 30 and 50 ppm. The total oxygen content includes, in addition to the free oxygen, the deoxidation inclusions present in the molten steel at the introduction of the molten steel into the casting pool. The free oxygen is formed into solidification inclusions adjacent the surface of the casting rolls during formation of the metal shells and cast strip. These solidification inclusions are liquid inclusions that improve the heat transfer rate between the molten metal and the casting rolls, and in turn promote the formation of the metal shells. The deoxidation inclusions also promote the presence of free oxygen and in turn solidification inclusions, so that the free oxygen content is related to the deoxidation inclusion content.
The low carbon steel may have a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.01% to 2.0% by weight and a silicon content in the range 0.01% to 10% by weight. The steel may have an aluminum content of the order of 0.01% or less by weight. The aluminum may for example be as little as 0.008% or less by weight. The molten steel may be a silicon/manganese killed steel.
The oxide inclusions are solidification inclusions and deoxidation inclusions. The solidification inclusions are formed during cooling and solidification of the steel in casting, and the deoxidation inclusions are formed during deoxidation of the molten steel before casting. The solidified steel may contain oxide inclusions usually comprised of any one or more of MnO, SiO2 and Al2O3 distributed through the steel at an inclusion density in the range 2 gm/cm3 and 4 gm/cm3.
The molten steel may be refined in a ladle prior to introduction between the casting rolls to form the casting pool by heating a steel charge and slag forming material in the ladle to form molten steel covered by a slag containing silicon, manganese and calcium oxides. The molten steel may be stirred by injecting an inert gas into it to cause desulphurization, and then injecting oxygen, to produce molten steel having the desired total oxygen content of at least 70 ppm, usually less than 250 ppm, and a free oxygen content between 20 and 60 ppm in the casting pool. As described above, the total oxygen content of the molten steel in the casting pool may be at least 100 ppm and the free oxygen content between 30 and 50 ppm. In this regard, we note that the total oxygen and free oxygen contents in the ladle are generally higher than in the casting pool, since both the total oxygen and free oxygen contents of the molten steel are directly related to its temperature, with these oxygen levels reduced with the lowering of the temperature in going from the ladle to the casting pool. The desulphurization may reduce the sulphur content of the molten steel to less than 0.01% by weight.
The thin steel strip produced by continuous twin roll casting as described above has a thickness of less than 5 mm and is formed of a cast steel containing solidified oxide inclusions. The distribution of the inclusions in the cast strip may be such that the surface regions of the strip to a depth of 2 microns from the outer faces contain solidified inclusions to a per unit area density of at least 120 inclusions/mm2.
The solidified steel may be a silicon/manganese killed steel and the oxide inclusions may comprise any one or more of MnO, SiO2 and Al2O3 inclusions. The inclusions typically may range in size between 2 and 12 microns, so that at least a majority of the inclusions are in that size range.
The method described above produces a unique steel high in oxygen content distributed in oxide inclusions. Specifically, the combination of the high oxygen content in the molten steel and the short residence time of the molten steel in the casting pool results in a thin steel strip with improved ductility properties.